1. Field of the Invention
The present invention relates to a method for forming an image of high resolution by transferring an image from a transfer material onto the surface of an image-receiving material using laser light.
2. Description of the Related Art
Heretofore, a system of directly pressurizing and heating a heat transfer material with a thermal head has been applied as a method for recording an image by heat transfer. This system has superior characteristics in that it is a maintenance-free dry process that can be implemented with low noise and simple device structure. The thermal head itself has become more highly dense and, even in this system, image resolution has been achieved at considerably higher levels in recent years.
On the other hand, a laser heat transfer recording system of irradiating a transfer material with laser light, converting the laser light into heat in the transfer material and conducting heat-sensitive recording by said heat has been known as a method of recording an image of higher resolution by heat transfer. In such a system, the laser light, being a source of supplying energy, can be collected to within several microns, thus enabling a significant improvement in resolution compared with the system using the thermal head.
In the field of graphic art, a printing plate is printed by use of a set of color separation films prepared from a color original using litho-film. Generally a color proof is prepared from the color separation films in order to check for errors in a color separation step, for necessity of color correction before printing (actual printing operation), and the like. In the color proof, there is a need for realization of a high resolving power capable of good reproduction of a halftone image and for characteristics such as high process stability. Further, to obtain a color proof resembling an actual print, the materials used in the color proof are preferably those materials used in the actual printing; for example, it is preferable to use regular printing paper as a support and pigment as a coloring material. Further, a dry process using no developing solution is strongly desired as the method of preparing a color proof.
As the dry process for preparing a color proof, a recording system of preparing a color proof directly from digital signals has been developed in accordance with the recent spread of electronic systems for pre-processes in printing (the preliminary press field). The object of such an electronic system is to prepare a color proof of particularly high quality, generally to reproduce a halftone-point image of at least 150 lines/inch. For recording a high-quality proof from digital signals, a laser light that can be modulated with digital signals and a device capable of narrowing down a recording light are used as a recording head. Accordingly, it is necessary to develop a recording material showing high recording sensitivity toward laser light and showing high resolving power so as to be capable of reproducing halftone points highly accurately.
As the transfer material used in the laser heat transfer recording system, a heat-fusion transfer material is known that has, on a support in this order, a light-heat exchange layer that absorbs laser light to generate heat and an coloring material layer that has a pigment dispersed in components such as heat-fusion wax, a binder and the like (Japanese Patent Application Laid-Open (JP-A) No. 5-58045 and the like). When such a heat-fusion transfer material is used, heat generated in a laser light-irradiated region of the light-heat exchange layer causes the coloring material layer corresponding to that region to be melted and transferred onto an image-receiving material that has been superposed on the heat-fusion transfer material, thereby forming a transferred image on the image-receiving material.
JP-A No. 6-219052 describes an image-forming method which includes forming a highly accurate image on an image-receiving material superposed on a transfer material that has a light-heat exchange layer containing a light-heat exchange material, a very thin (0.03 to 0.3 xcexcm) thermally releasable layer, and a coloring material layer containing a coloring material arranged in this order on a support. In this method, a binding force between the coloring material layer and the light-exchange layer, which are bound via the thermally releasable layer, is reduced by irradiation with laser light. In this image-forming method, so-called xe2x80x9cabrasionxe2x80x9d is utilized. Specifically, the thermally releasable layer is partially decomposed and gasified in a region irradiated with laser light, thus weakening the binding force between the coloring material layer and the light-heat exchange layer in that region, and permitting the coloring material layer in that region to be transferred onto the image-receiving material superposed on the transfer material, thereby forming a transferred image on the image-receiving material.
An image-forming method utilizing laser light has advantages such as usability of regular printing paper provided with an image-receiving layer (adhesive layer) as the image-receiving material, easy provision of a multi-color image by successive transfer of images of different colors onto the image-receiving material, and the like. In particular, an image-forming method utilizing abrasion has the advantage of easy provision of a highly accurate image, and is particularly useful for forming a color proof (direct digital color proof (DDCP)) or a highly accurate mask image.
In these methods of forming an image by heat transfer, the resolution of the image is significantly influenced by adhesion between the transfer material and the image-receiving material during transfer of the image. Thus, increasing the adhesion between the two is a key to achieving high-resolution images.
As a method of increasing the adhesion between the transfer material and the image-receiving material, a method in which the transfer material and/or the image-receiving material is provided with a cushion layer endowed with flexibility, to absorb unevenness between the two, has been disclosed (JP-A No. 5-169861).
However, if the image-receiving material is provided with the cushion layer, it is hard to use the image-receiving material as a final recording medium. Thus a step of transferring an image formed on the image-receiving material onto a final recording medium is necessitated, which causes the problem of a complicated process. Further, if the transfer material is provided with the cushion layer and the image-receiving material is used as a final recording medium, there will be cases where unevenness of the surface of the final recording medium cannot be sufficiently absorbed by the cushion layer. Thus, there is a need for an image-forming method capable of forming a high-quality image with high resolution even if adhesion between the transfer material and the image-receiving material is not guaranteed.
Further, in recent years, in order to reduce recording time when recording an image with laser light, a laser light consisting of a multi-beam two-dimensional array, in which a plurality of laser beams is used, has been used. When recording on a conventional heat transfer sheet with a laser light in a multi-beam two-dimensional array, there are cases where the density of the transferred image formed on the image-receiving sheet becomes insufficient. In particular, the density of the image is significantly reduced in the case of laser recording with high energy. As a result of investigations, by the inventors of the present invention, it was found that this reduction in the density of the image is caused by uneven transfer occurring in laser recording with high energy.
Accordingly, an object of the present invention is to provide an image-forming method capable of forming a transferred image having high resolution, high quality and good color tones, without parts missing from the image, even if a transfer material and an image-receiving material are not provided with a cushion layer.
Another object of the present invention is to provide a method for forming a multi-color image which method can form a high-quality image with stable transfer density on an image-receiving sheet, even if laser recording is conducted with high-energy by a laser beam in a multi-beam two-dimensional array under various temperature and humidity conditions.
A first aspect of the invention is a method for forming an image, the method comprising the steps of: charging, by corona discharge, a coloring material layer surface of a transfer material, which has at least a light-transmissive support, a light-transmissive electroconductive layer, a light-heat exchange layer and a coloring material layer; thereafter superposing an image-receiving layer surface of an image-receiving material, which has at least a support and an image-receiving layer, with the coloring material layers surface; and irradiating laser light imagewisely onto the transfer material, thereby transferring an irradiated portion of the coloring material layer of the transfer material to the image-receiving layer surface, for forming the image on the image-receiving layer surface of the image-receiving material.
A second aspect of the invention is a method for forming a multi-color image, the method comprising the steps of: providing an image-receiving material, which has an image-receiving layer, and four transfer materials, of yellow, magenta, cyan and black, each of which has, on a support, at least a light-heat conversion layer and a coloring material layer; opposing and superposing the coloring material layer of each transfer material with the image-receiving layer of the image-receiving material; and irradiating laser light, which is a multi-beam 2-dimensional array of laser beams, from a support side of the each transfer material, thereby transferring a laser-light irradiated region of the coloring material layer of the each transfer material onto the image-receiving layer of the image-receiving material, for recording the image, wherein layer thickness of the coloring material layer of the black transfer material is greater than layer thickness of the coloring material layer of each of the yellow, magenta and cyan transfer materials, and the layer thickness of the coloring material layer of the black transfer material is from 0.5 to 0.7 xcexcm.